opp: Fix return in _opp_add_static_v2()

Fix sparse warning:
drivers/opp/of.c:924 _opp_add_static_v2() warn: passing zero to 'ERR_PTR'

For duplicate OPPs 'ret' be set to zero.

Fixes: deac8703

 ("PM / OPP: _of_add_opp_table_v2(): increment count only if OPP is added")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>